Viscous fluid couplings have received wide acceptance in the automotive industry for controlling the amount of torque transmitted to a radiator cooling fan. The most common form of such viscous fluid couplings is the air temperature response type such as illustrated in U.S. Pat. No. 3,055,473. In certain applications, however, it has become desirable to directly sense the engine cooling water temperature rather than temperature of the air passing through the radiator. To date, many arrangements have been proposed to accomplish this result. Typically, these arrangements have made use of wet or dry plate clutches that are pneumatically actuated or electromagnetic clutches that are electrically actuated. A major shortcoming of many prior art magnetically actuated clutches or couplings resides in the fact that relatively high torque levels are involved. These high torque levels require relatively massive engagement mechanisms which must be engaged through the action of relatively intense magnetic fields set up by large and expensive inductors. Such inductors draw substantial amounts of energy from the associated vehicle's electrical system and thus reduce overall operational efficiency. Additionally, such devices typically employ relatively large springs which bias a portion of the engagement mechanism toward either the engaging or nonengaging position. Accordingly, the magnetic field must also overcome the force of the biasing spring as well as the mass of the engagement mechanism.
More recently, improved prior art electromechanically actuated viscous fluid couplings have been suggested which employ inductors to establish magnetic fields which actuate valves controlling the flow of viscous fluid within the clutch. This arrangement represents an improvement inasmuch as the magnetic field established by the inductor only had to be large enough to move a relatively small valving arm and biasing spring. Although representing an improvement, such devices had inherent inefficiencies inasmuch as their valving arm biasing springs had to be large enough to overcome the kinetic energy of the viscous fluid flowing therein.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized with fluid couplings intended for many different applications. However, the invention is especially useful when applied to a viscous coupling or clutch which serves as a drive for a radiator cooling fan of a vehicle engine, and will be described in connection therewith.